Doing the right thing? More harm than good?

My mother told me two weeks ago that my younger brother who is now 40 years old is not my dad's son. I've suspected this over the years, purely by looks. This makes no difference to me at all, he's my brother and that's that. My mum now constantly talks of a huge guilt trip regarding my brother (who has not been told anything) and his biological father who was denied any contact. My own father had brought myself and my two brothers up equally and lovingly. The story goes my mum had an affair about 42 years ago. She became pregnant, my dad found out and she was due to start a new life with her new man and my then baby brother with a view to myself and my older brother joining her once they had found a place to live. My mum stayed at her father's house for about 8 weeks back in 1970 - I remember this clearly - I was 7 at the time and stayed with my dad and my brother in the famly home. A long story short my mum and my baby brother came back home and life carried on as if nothing happened. We children had no idea of the circumstances. My mum says it broke her heart but she had to choose between a new life with her new man and my little brother or come back home for fear of losing myself and my older brother. Finances were a big consideration also. Her man had just lost his job. Consequently she wrote him a letter explaining and never heard from him again. Many years on she phoned him and he was angry with her and said he had waited 10 years for her to get in touch. Apparently he married some years later and settled down and had children with his new wife. Question/Advice please: Does she tell my brother the truth after 40 years - we don't know if he has any clue. Should we try and contact his bio father - I don't want to upset his life and family. I don't want to be disloyal to my dad. My mum is crying out for help for me to locate the man from the past to check that he would want anything to do with my brother before we disclose anything. He will be early 70's now, if he is indeed still alive. My older brother says people will get hurt and let sleeping dogs lie. Another half of me thinks my younger brother should know the truth about his roots but at what cost. Thank you anyone for any input x